It is known to coat the interior of metal pipes by forcing entrained particles of plastic to flow through a preheated, rotating pipe. DeHart, U.S. Pat. No. 3,207,618, discloses apparatus and method by which a fluidized bed of plastic particles are passed into a pipe in order that the particles may adhere to the interior sidewall thereof and bond to one another and to the surface of the pipe, to provide a continuous coating. The Dehart disclosure suggests duplicate equipment arranged at opposed ends of the pipe so that the entrained plastic can be flowed through the pipe in a first direction and thereafter flowed through the pipe in a reverse direction, thereby providing a more uniform coating on the interior of the pipe. Moreover, DeHart employs a separator and a vacuum combination at the outflow end of the pipe.
Stallard, U.S. Pat. No. 3,532,531, is similar to the DeHart disclosure, and additionally supercools the plastic particles prior to forcing the particular plastic to flow into the rotating pipe.
Blackburn, U.S. Pat. No. 2,919,160, discloses apparatus for dispending powdery materials wherein a fluidized bed of pulverulent material is transferred to a mold.
Condo et al, U.S. Pat. No. 3,814,616, places a pipe to be coated within an oven and coates the interior thereof by sucking air into one end while feeding negatively charged, dry particles of a coating composition into the other end.
Randell, U.S. Pat. No. 2,758,546; Dalley et al, U.S. Pat. No. 1,997,761; DeHart, U.S. Pat. No. 3,207,618; Star et al, U.S. Pat. No. 3,208,869; Church, U.S. Pat. No. 3,108,022; and Weidenhammer et al, U.S. Pat. No. 3,260,611, are further examples which set forth known prior art expedients which involve the handling of particulated plastic and the deposition of the plastic onto a wall surface in order to form a coating. Reference is made to the above issued patents, and to the art cited therein, for further background of this invention.
In actual practice, so far as Applicant can determine, these and other presently known processes for coating the interior of metal pipe fail to provide a coating of substantial uniform thickness. It is for this reason that some processes require that the pipe be twice treated by flowing the plastic material in one direction through the pipe, and thereafter flowing the material through the pipe in an opposite direction, thereby laying down two separate coatings in order that the thickness of the coating will not unduly diminish from one end to the other of the pipe. These and some other known processes require reprocessing as much as 30 percent of the completed pipes because the coating is unsatisfactory.
After a pipe has been coated and tested, should the coating fail to measure up to acceptable standards, the coating must be removed and the pipe returned for reprocessing. Usually the coating is burned out or oxidized by utilizing a flame on the interior thereof. The reprocessing of unsuitable pipe is expensive.
It would therefore be desirable to be able to apply a protective plastic coating to the interior of the pipe in such a manner that the coating is uniform from one end of the pipe to the other and about the entire inside peripheral surface of the pipe, while at the same time, the process of coating is carried out in such a manner that very few of the pipes must be reprocessed.